(1) Field of the Invention
The present invention relates to the optical design and microelectronic fabrication of microlens arrays for the optimization of spectral collection efficiency and related photodetector signal contrast in the color filter process of semiconductor imaging devices.
(2) Description of Prior Art
Image sensors for color digital still and analog or digital video cameras are typically charge-coupled device (CCD) or complementary metal-oxide semiconductor (CMOS) photodiode array structures which comprise a spectrally photosensitive layer below one or more layers comprised of an array of color filters and a plane-array of microlens elements. The elementary unit of the imager is defined as a pixel, characterized as an addressable area element with intensity and chroma attributes related to the spectral signal contrast derived from the photon collection efficiency of the microlens array, spectral transmission or reflection properties of the color filter array materials, and spectral response and electrical noise characteristics of the photodetectors and signal processing train.
The ability of a sensor to capture images in low-level irradiance conditions is critical in applications. The primary attributes of the sensor that determine its ability to capture low-level image light are the geometrical optics of the lens arrangement, fill-factors of lenses and photodiodes, and, the photoelectron quantum efficiency of the semiconductor in which the photodiodes are fabricated. The quantum efficiency is a measure of the photon-to-electron conversion ratio, and, for most CCD""s these quantum efficiencies are similar. But, the physical size of the photosensitive area, coupled with the geometry of the lenses for collecting light and imaging this light onto the useful photosensitive area, create superior or inferior solid-state imagers. Responsivity is a measure of the signal that each pixel can produce and is directly proportional to pixel area. Another benefit of increased responsivity is that less illumination is needed to achieve a desired signal- to-noise contrast. With low-level illumination, the image will appear less grainy, and, the imager""s frame rate can also be increased, providing increased video rates.
The optical performance of a solid-state imager is seen to depend on pixel size. Pixel size also affects the interaction of the camera lens with the microlens array. The microlens on top of each pixel focuses light rays onto the photosensitive zone of the pixel. The microlens significantly increases responsivity but it also limits the angular range of good responsivity. Typically, a 5-micron pixel has a severe drop in responsivity at 5 degrees from normal incidence. This leads to an optical effect whereby pixels near the edge of the camera lens field of view collect light less efficiently than at the center. This effect is reduced with larger 9-micron pixels, which have high responsivity at angles out to 15 degrees or more. Truncation of illuminance patterns falling outside the microlens aperture results in diffractive spreading and clipping or vignetting, producing nonuniformities and a dark ring around the image. Larger pixels use more silicon area which drives up the solid-state imager device manufacturing cost. The size of the active area can be set to optimize three factors: low light sensitivity, overall sensor size, and, the size of the optics necessary to project the desired image over the entire array. Instead of increasing the active area, some sensor manufacturers add extra steps to the manufacturing process to apply a microlens over each pixel. A microlens captures most of the incident light and focuses it onto the active area, which increases the effective fill ratio. The trade-off is, therefore, between the added cost of the microlens processing steps and the cost of the larger active areas. Typically, a pixel with a microlens requires a narrower incident light angle than a pixel that does not use a microlens, imposing additional optical design implications for the lens of the camera.
Associated with the microelectronic fabrication processes and materials used in forming solid-state array imagers, there often results gradients of spectral sensitivity and/or responsivity across the individual sensor elements comprising the imaging matrix, as well as related variations from sensor to sensor. The intrasensor variation and the intersensor variation are convolved with the imaging optics of the camera and the microlens arrays overlaying the photodiode arrays, such that the resultant contrast or modulation transfer function which manifests itself as an optoelectronic signal distribution mapping the illuminance distribution of the image formed across the imager-array surface, is an electronic signal contrast function which is fed into an electronic signal processing train which again further convolves noise and other contributions into the output of the imager. In particular, the regions within and between pixels which suffer significant variations ranging from peak sensitivity and responsivity to xe2x80x9cdead zonesxe2x80x9d of practically no photosensitivity, or, vignetted light from the optics which truncated and diffracted the original image light-intensity distribution function, direct the attention of solid-state imager designers to improve the geometrical optical configurations to optimize light collection to focus images onto active areas. More particularly, in cases where sensing areas are formed into non-regular or L-type geometries, conventional microlens designs typically yield relatively low light collection efficiencies and result in degraded optoelectronic signal contrast. Images formed may suffer from known types of classical lens aberrations, produce motional smearing, pixel gaps, or, have other significant undesirable effects on image quality. Practical, manufacturable imaging arrays must, therefore, be designed to increase flexibility in the layout of the imaging device. Central to this goal is the optimization of the design of the multi-microlens array configuration geometry and the formation process steps for the microelectronic fabrication of these optics overlaying the photodiode elements in the matrix comprising the solid-state imager. Variations in lens curvature, index of refraction, and, light-scattering centers in the materials which are caused to flow to form the microlens arrays will combine to determine the resultant image quality and signal strength. Unless the microlenses are ideally diffraction- limited, imager defects will always be introduced in practical situations.
The design and fabrication of the multi-microlens arrays over the microelectronic photosensors of the solid-state imager, with enhanced optical collection efficiency for non-regular and L-shaped sensing areas, is thus a goal to which the present invention is directed. In conventional configurations for the color-filter process, only one microlens covers one pixel, and, the center of the microlens is aligned to the center of the pixel. It is, therefore, another goal of the present invention to disclose multi-microlens structures and configurations suitable for L-shaped and non-regular sensing areas applicable to manufacturing CMOS and CCD image devices. Park et al in U.S. Pat. No. 5,877,040 shows a CCD with a convex microlens formed integrally on the planarization layer above a photodiode element of a CCD array such that the focal-distance of the lens may be positioned by adjustment of the microelectronic fabrication process of intervening film-layer thicknesses. By fixing the irradiance pattern comprising the image on only the photodiode, image smear caused by the photoelectric effect when diffracted peripheral light is incident on the interpixel regions is said to be minimized. The microlens formation process described is derived from the flow of a microlens material which flows thermally at 100 degrees to 200 degrees Centigrade. Following a dry-etch step of the substrate, the set of convex microlenses are formed in a second planarization layer above a first planarization layer containing color filters. The light receiving area of the photodiode is increased as compared with the conventional CCD because the photodiode is formed in an overlay with the microlens. However, rheological and surface physical processes, such as those taught by Park et al, wherein flow and surface tension are required to form a matrix of convex microlenses, are not consistent with the formation of irregular shapes to overlay to irregular or L-shaped photodiode geometries. Similarly, Song et al. in U.S. Pat. No. 5,672,519 teaches an image sensor device with a compound regular-shape microlens which extends conventional prior art from square to rectangular illuminance areas to account for CCD structures where the dimensions of a unit pixel or photodiode are different in the vertical and horizontal directions of the semiconductor. Song et al accomplish their extension of the prior art by two successive iterations of the conventional melt and flow process to cascade a contiguous upper lens of different curvature and/or refractive index on a first, lower lens to accommodate the dimensional mismatch of the pixel (photodiode). The fabrication method consists of forming lens shapes by carrying out patterning of transparent photoresist having a refractive index of 1.6 and melting it to cause flow which, under surface tension, results in a mosaic of hemispherical convex lense array elements. Simple convex and compound convex lens classes representative of Prior Art are shown in FIG. 1(a), FIG. 1(b). and FIG. 1(c). In FIG. 1(a), a first hemisherical lens element 1 of a two-dimensional array of microlenses is formed in the manner described above. A successive polymer, resin, or photoresist film coating is conformally applied, photolithographically patterned, thermally reflowed and/or dry-etched. The second-layer photomask and thermal and surface tension conditions of the first microlens 1 array-plane determine the curvature and thickness of the second tandem microlens array-elements 2. The compound lens in FIG. 1(a) is shown comprised of first lens 1, second lens 2, forming light-cone 3 converging to focal point or area 4. FIG. 1(b) shows a planarized first lens 5 with second lens 6 forming a compound lens converging image light-cone 7 to focal area 8. In FIG. 1(c) first microlens 9 is again planarized but also faceted by etching, and second lens 10 is conventionally formed to comprise the compound lens to converge image light-cone 11 to focal area 12. Curvature control is difficult even for a single fabrication step, and, all the determinents of variance that apply to the single step apply a fortiori to the iterated process forming the compound lens. The final structure achieved by Song et al produces parallel stripes of microlenses across a base mosaic of microlenses, which can be planarized to provide a flat surface for the second lens array-plane. In all cases of the Prior Art it is observed that single simple or single compound microlens arrays, with a mapping of the single microlens, simple or compound, to a single pixel or single photodiode sensing area in the imager. The limiting numerical aperture or f-stop of the imaging camera""s optical system is that of the smallest aperture element in the convolution train. Therefore, it is observed that the Prior Art represented in FIGS. 1(a) and (c) have in common the further limitation of the light collection capability of the larger numerical aperture first (lower) microlens element by the addition of the second tandem microlens element (upper) which preceeds it.
Spherical aberrations, coma, light scattering, numerical aperture variations, vignetting, reflective losses at interfaces, multibounce stray light, cross-talk and other optical defects described by the modulation transfer function of the upper lenses are convolved with the lower or base lenses modulation transfer functions having their own analogous set of defects. L-shaped and irregular-shaped pixels remain problems not solved by the methods taught by Song et al.
U.S. Pat. No. 5,336,367 to Nomura attempts to eliminate flicker and other image defects by constraining the color-filter layer distance to the light sensing element, avoid color filter cracks due to thermal impact and enhancing reliability by creating the color-filter using an etch-resistant layer as a mask, and, forming a mosaic array of microlenses on the etch-resistant layer. The manufacturing process described is limited to forming hemispherical convex lenses and, therefore, subject to the same set of limitations of the conventional structures and fabrication processes described herein.
FIG. 2(a) depicts conventional geometric relationships between a square pixel 13, aperture 14 of a microlens element orthogonally projected onto a pixel, active sensing area 15 of a photodiode, and an area of a circle 16 of a microlens focal xe2x80x9cpointxe2x80x9d for converging incident light to fall onto the center of a pixel in the plane of a photodiode. FIG. 2(b) illustrates the geometrical optics for incident image light 16 converged by microlens element 17 into focal cone 18 to the focal area 19 within a photoactive area 20 surrounded by a dead or non-photosensitive area 21. FIG. 3(a) and FIG. 3(b) illustrate the case where the photosensing area is a non-regular geometry. Shown in FIG. 3(a) is a square pixel 22 in which is inscribed the aperture stop 23 of the microlens which has focussed the incident image light into focal area 24 in the gap between the split non-regular sensing area 25. FIG. 3(b) exhibits the geometrical optics for incident image light 26 converged by microlens element 27 into focal cone 28 to the focal area 29 in proximity to a non-regular photoactive area 30. FIG. 4(a) and FIG. 4(b) illustrate the case where the photosensing area is L-shaped. Shown in FIG. 4(a) is a square pixel 31 in which is inscribed the aperture stop 32 of the microlens with focal cone cross-sectional area 33 in the L-shaped photodiode area 34. FIG. 4(b) shows the incident image irradiance 35 focussed by microlens 36 into focal cone 37 converging at focal area 38 onto or in proximity to the L-shaped photosensing area 39.
A principal object of the present invention is to provide classes of multi-microlens optical structures, configurations, arrangements, and combinations with spectral color-filters that maximize collection-efficiency of optical radiation patterns of images superimposed on the surface of semiconductor array imaging devices with non-regular and L-shaped sensing areas to achieve optimum color pixel resolution.
Another object of the present invention is to provide a manufacturable method and fabrication process which minimizes the number and task-times of the operational steps required in the production of semiconductor array color-imagers in accord with the principal object of the present invention.
Another object of the present invention is to provide multi-microlens design structures and fabrication methods that are practical for high reproducibility, reliability, and, consequently maximum yield and minimum production cost.
A further object of the present invention is to increase flexibility of the design and layout of color-imaging devices.
In accordance with the objects of this invention, classes of multi-microlens optical constructs which can be combined with either conventional or novel color-filter fabrication sequences for CCD,CID and CMOS array imaging devices are taught. To practice the method of the present invention, conventional microelectonic fabrication techniques using photolithograhic materials, masks and etch tools are employed: in succession the array of pn-junction photodiodes is patterened with impurity dopants diffused or ion-implanted, isolated, planarized over, and, typically three more layers are built up additively with primary red, green, blue color-filters formed by the addition of suitable dyes appropriate to the desired spectral transmissivity to be associated with specified photodiode coordinate addresses in the imager matrix.
Single chip color arrays typically use color filters that are aligned with individual columns of detector elements to generate a color video signal. In a typical stripe configuration, green filters are used on every other column with the intermediate columns alternatively selected for red or blue recording. Various combinations and permutations of color-filter sequences or color-filter xe2x80x9cmasksxe2x80x9d are possible, and, a number of microprocessor algorithms exist for balancing color components in the synthetic reconstruction of color images.